Unfortunately, I'm not the author and I had never used it so you should contact directly the author about that.
Former userAccount Deleted
I'm also having problems with the the S3 backend with the latest django trunk. I've opened a ticket on the django trac as well ( http://code.djangoproject.com/ticket/10300 ). The problem seems to start at django svn r9766.
David Larletrepo owner
Does attached patch in Django's tracker solve the MogileFSStorage issue too?
Not sure about MogileFSStorage, I haven't used it. But it does seem to do the trick for S3.